Non-contact vibration measurement using laser doppler vibrometry is a well-established technique. The laser doppler vibrometry method uses an interferometer to measure the doppler frequency shift induced by the vibration of the object. In order to measure the vibration of the object, the measurement beam strikes the object to be measured and interferes with a reference beam. The resulting frequency shift induced in the interference beam is the vibration of the target surface. The measured vibration signal is the average vibration of the object over the entire beam diameter of the measurement beam striking the object.
U.S. Pat. No. 4,554,836 (Rudd), U.S. Pat. No. 5,394,233 (Wang), U.S. Pat. No. 5,838,439 (Zang et al.), U.S. Pat. No. 5,883,715 (Steinlechner et al.), etc., describes single point vibration measurement using laser doppler vibrometry. Some applications require scanning the beam over the object surface in order to measure the dynamic parameter of the object over an area rather than at a single point for better analysis. Moving the object stage in the X-axis and Y-axis and keeping the beam at a fixed point will allow measurement of the vibration over an area. But such a means for scanning the beam over the object is limited by the accuracy of the moving stages, and in most application it is not possible to move the object. Moreover, the motion of the stage creates vibration and increases the noise. In which case, the object of measurement should be fixed, and the beam needs to scan from one point to the next.
Laser doppler vibrometer can also be used to measure the flying height, i.e., the distance between the slider head and the disk surface in a hard disk. The flying height is a critical parameter and needs to be measured accurately in order to assure optimal performance. Methods such as capacitance, monochromatic interferometry, and white light interferometry are other methods which can also be applied to the measurement of flying height. All the interferometric measurement techniques involve measurement of flying height at a single point or by scanning the beam by moving the hard disk in the X-axis and the Y-axis. This process will not lead to accurate measurement of flying height due to the error induced by the mechanical movement of the hard disk. Also, the measured flying height is the average of the overall flying height of the area of the measurement spot, which is rather large in all these system. Therefore, the smaller the spot size the more accurate is the measured flying height.